A synthetic substance has been shown to improve the rate of survival and lessen complications associated with respiratory distress syndrome in certain premature babies, according to a study that appears in today's edition of the New England Journal of Medicine.

Known as Exosurf Neonatal, the substance was shown to be effective in treating larger preemies -- those who weigh more than 1250 grams, or 2 lbs., 12 ozs. The findings are significant, the authors said, because half of all premature babies who experience RDS fall in that category.

The synthetic substance, or surfactant, keeps immature lungs of preemies with RDS from collapsing. A freeze-dried powder and sterile water mixture, it is administered through an infant's ventilator tube.

RDS affects about 50,000 babies each year and is a leading cause of death and disability among premature infants, who lack their own surfactant.

"While the efficacy of synthetic surfactant has been widely tested in small premature infants, its effects on larger preemies had remained largely unexplored," said Dr. Walker Long, senior clinical research scientist at Burroughs Wellcome Co., of Research Triangle Park, N.C., and one of the authors.

The study is the largest placebo-controlled surfactant study ever conducted, involving 1,237 infants at 36 institutions throughout the United States and Canada. Of the total, 623 infants were randomly assigned to a placebo and another 614 were randomly assigned to Exosurf Neonatal.

The results showed that in the first 28 days of life there were fewer deaths due to RDS, lower overall neonatal mortality, lower incidence of bleeding in the brain and a lower incidence of chronic lung disease among those treated with Exosurf Neonatal.

The study also determined there were dramatic reductions in the frequency of a number of other complications of RDS and prematurity, such as lung rupture, seizures, low blood pressure and meningitis.

The authors also said that infants who received surfactant spent significantly fewer days on oxygen, mechanical ventilation and any form of respiratory support.

"Withholding surfactant from larger infants with RDS who are receiving ventilation -- because they are thought to have good prognosis without such treatment -- is not justifiable," Long said.

Surfactant therapy was cited by the National Center for Health Statistics as contributing to a 6 percent decline in deaths among premature infants in 1990.